1. Field of the Invention
The present invention relates to a power working machine such as a backpack power duster or blower apparatus and, more particularly, is concerned with a centrifugal blower employed in a portable power working machine.
2. Related Background Art
In portable power working machines using a centrifugal blower, various means for reducing noise generated from the centrifugal blower have conventionally been suggested. For example, Japanese Utility Model Publication No. SHO 62-40159 discloses a centrifugal blower having a sound absorbing material disposed adjacent a volute chamber. Also, U.S. Pat. No. 5,857,439 discloses a backpack power working machine having a sound absorbing panel attached to a backpack frame so as to surround a centrifugal blower.
While these conventional techniques can lower the noise generated from the centrifugal blower to a certain extent, there is always a demand that such noise should be reduced as much as possible.
The inventor has carried out various studies in order to fulfill the above-mentioned demand and, as a result, has found that, if individual blades of an impeller rotate along the inner wall surface of a casing, then a compression wave of air occurs between the impeller and the inner wall surface of the casing, thereby becoming a major factor for the noise.
Therefore, one aspect of the present invention features a centrifugal blower comprising an impeller having a hub plate rotating about an axis, a set of blades disposed on at least one surface of the hub plate so as to surround the axis, and an air inlet defined by inner ends of the blades, the impeller taking air into the air inlet upon rotation so as to discharge the air radially outward; a casing surrounding the impeller, the casing having an intake port communicating with the air inlet and having a volute portion disposed at an outer peripheral portion thereof; and sound absorbing means disposed at a portion of the casing facing the set of blades.
With this arrangement, the noise caused by the compression wave of air occurring between the set of blades of impeller and the inner wall surface of casing can efficiently be absorbed by the sound absorbing means adjacent the set of blades. Also, while the inventor has found that the noise caused by the compression wave of air increases as the gap between the inner wall surface of casing and the set of blades is smaller, such an increase in noise can be avoided in accordance with the present invention even when the inner wall surface of casing is disposed closer to the set of blades. As a result, the blowing power of the centrifugal blower can be enhanced without increasing the noise. It is because of the fact that the loss in energy decreases as the gap between the inner wall surface of casing and the set of blades is smaller, thereby enhancing the blowing power.
Though the sound absorbing means is preferably disposed over the whole area facing the set of blades, it may be disposed at a part of this area.
Preferably, the sound absorbing means comprises a wall portion of the casing, adjacent and opposite the set of blades, formed with a plurality of through holes, and a sound absorbing material disposed on an outer surface of the wall portion.
Another aspect of the present invention features a power working machine comprising a centrifugal blower such as the one mentioned above. Such a power working machine is a backpack power duster or backpack blower apparatus, for example.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.